Ultra short wave tube



April 14, 1936. P, D. ZOTTU ULTRA SHORT WAVE TUBE Filed May 29, 1934 INVENTOR I PAUL D. ZOTTU.

W ATTORNEY Patented Apr. 14, 1936 ULTRA SHORT WAVE TUBE Paul D. Zottu, West Caldwell, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application May 29, 1934, Serial No. 728,120

3 Claims.

My invention relates to a high frequency generator of the thermionic vacuum tube type for generating ultra short electric waves, of a length less than one meter, and more particularly to a thermionic tube suitable for use in the generation of high frequency oscillations for producing such waves.

With the vacuum tubes now in general commercial use the shortest waves that can be obtained with regenerative coupling have a length of approximately one meter and are usually so weak that they are useful principally for measuring purposes. Although shorter Waves can be produced with a three electrode vacuum tube by the Barkhausen and Kurz method, the type of tube which has heretofore been generally used to generate ultra high frequency oscillations is the cylindrical type with a straight cathode surrounded by a helical grid electrode and a tubular plate electrode. With such a tube short waves can be produced by applying a negative potential of about ten to twenty volts to the tubular electrode and a positive potential of a few hundred volts to the helical grid-like electrode, which then becomes the collector or anode. By increasing the potentials, or by reducing the electrode spacing, shorter waves can be obtained, but the extent to which this can be done in the cylindrical type of tube heretofore used is limited by various practical difficulties. Another difficulty with this type of tube has been that it usually generates waves of several different lengths simultaneously. A 'flat type of tube with a cathode, such as a V or M shaped filament, with its strands in a common plane, and other plane electrodes; one of which is a grid-like electrode with its conductors transverse to the cathode strands, parallel to the plane of the cathode, has also been used to produce ultra short waves, but in these flat type tubes, as heretofore made, it has been difficult to get the very close electrode spacing necessary for obtaining very short waves.

One object of my invention is to provide an ultra short wave tube for generating oscillations which, so far as can be determined by the commonly used method, are of only one fundamental frequency.

Another object of my invention is to provide a short wave tube which is simple in construction, more efiicient in operation than the tubes which have heretofore been available, has extremely close spacing of the electrodes, and is capable of producing ultra short waves, such as waves having a length of the order of magnitude of ten cm. A further object of my inventionis to provide a cylindrical type of ultra short wave tube in which the spacing oftheelectrodes is closer than has heretofore been feasible, and the positive electrode has reduced inductance and increased heat dissipation.

In accordance with my invention I provide a short wave tube having a straight cathode and a positive grid-like electrode composed of two or more straight wires mounted parallel to the filament and spaced extremely close to the filament and to each other. Such a tube is more eflicient, simpler in construction and capable'of greaterheat dissipation than the tubes heretofore used in which the conductors of the gridlike positive electrode extend'transvers'ely of or at an angle to the cathode. By spacing the electrodes so that the spacing of the cathode to the other electrodes is such that the times of transit of the electrons on both sides of the positive electrode are substantially equal, I obtain a tube which oscillates at only one fundamental frequency. V p The novel features which I believe to be characteristic of 'my invention are set forth with particularity in the appended claims, but the invention itself will best" be understood by referenc'e to the following specification taken in connection with the accompanying drawing in which Figure 1 is a view in perspective, with some parts broken away, of one form of my invention embodied in a flat or parallel plane type of tube; Figure 2 a perspective view of a cylindrical type oftube with part of the tubular outer electrode broken away; and Figure 3 a cross-section of the tube shown in Figure 1 taken along the line 33; Figure 4 a longitudinal section of the cylindrical type of tube shown in Figure 2; Figure 5 an exploded view of the electrode assembly of the tube shown in Figure 1; and Figure 6 a diagram of one form of circuit in which a flat type of tube constructed in accordance with my invention may be used. I p 2 In the particular embodiment of my invention shown in Figure 1 the electrodesv of the tube are enclosed in a highly evacuated envelope or bulb I0 having at oneend theusual reentrant stem H which carries the electrodes. The mount of the tube shownin Figure 1 comprises two rectangular insulating sheets l2, preferably of mica, which carry the electrodes and which are mounted parallel to each other on a pair of side support rods l3 projecting from the stem II. The mica sheets l2 may be secured to the side "rods side between the mica sheets l2 which support the electrodes and hold them properly spaced.

The four electrode assembly, with the electrodes positioned as best shown in Figures 1 and 3, has a straight cathode l5, preferably of the filamentary type, fitted rather snugly into holes in the mica sheets and connected at the lower end to a cathode lead l6 in the stem l l and at the upper end to a cathode connector ll, preferably resilient, which holds the cathode taut. The cathode connector, which is preferably set normal to the plane of the side rods l3, has one end secured to the upper end of the cathode and the other end to the right hand support rod l 3, which is connected to a leading-in wire in the stem, and thus supplies current to the upper end. of the cathode. The positive grid-like electrode or anode has two straight rods or wires mounted parallel to the cathode and preferably fitting snugly into holes in the mica sheets. These two wires are preferably of about the same size as the cathode, and are spaced apart a distance about equal to the spacing between each rod and "the cathode, so that the cathode and the two rods are, as best shown in Figure 3, at the apices of an isosceles triangle. The lower ends of the two rods are connected, preferably through a sheet metal connector 19, which is below the lower mica sheet 12 and also acts as a heat radiator, to a lead 26 in thestem. It is convenient, though not necessary, to make the grid-like electrode l-8, as shown in Figures 1 and 5, in the form of a loop with parallel sides, and to hold it taut .by a resilient hook 2i engaging the bight of the loop, and mounted on the left-hand support rod 13 in the plane of the support rods. The intermediate portion of the grid-like electrode 18 consists solely of two wires parallel to and at least as long as the electron emitting portion of the oathode 15,1and spaced to leave between them an unobstructed gap ,in alignment with the cathode. The cathode l5 and the rod-like positive electrode l8 are, as best shown in Figures 3 and 5, mounted between and parallel to a pair of fiat sheet electrodes 2.2 and 23. The fiat electrode 22, or backing plate, is mounted behind the cathode and in conjunction with the straight cathode and produces in effect a flat cathode. The other flat electrode 23, or limiting electrode, is mounted in front of the positive electrode I8. These two fiat electrodes may be held in place between the two :mica sheets by wire fingers 24 secured to the electrodes and projecting into holes in the mica sheets. These flat electrodes are connected to .leads 25 which may conveniently be brought out through the top of the bulb, as shown in Figure 1. .A tube constructed as above described has been used to generate electric waves about ten cm. long. In this tube the flat electrodes 22 and 23 are about 250 mils long and about mils wide, and are spaced about 30 mils apart; the cathode I5 is a straight thoriated tungsten wire, activated to have high-emission, slightly longer than the flat electrodes, and about 2 mils in diameter; and the straight wires of the positive electrodes l8 are of tungsten, and of the same diameter as the cathode. The cathode l5 and positive electrode 18 are spaced equally from each other and from the fiat electrodes, as best shown in Figure 3, so thatthe electrode I 8 is midway between the oathode and the limiting electrode 23. With this tube, waves about ten cm. long and of considerable strength were obtained when the grid-like electrode "It was maintained at about volts positive, the limiting electrode 23 at a few volts negative, and the backing electrode 22 slightly negative, all with reference to the cathode.

The cylindrical type of tube shown in Figures 2 and 4 has the grid-like electrode of wires parallel to the cathode. The particular tube shown has a tubular electrode 26 which acts much like the limiting electrode 23 of the tube shown in Figure 1. Fitted into each end of the tubular electrode 26 is a ceramic plug 21 having a center hole and also a number of other holes arranged in a circle about the center hole. The grid-like electrode 28 consists of straight tungsten wires threaded through these other holes in the plugs 2'! and forming in effect a cylindrical grid-like electrode inside and concentric with the tubular electrode 26. The wires of the electrode 28 are all electrically connected in parallel, preferably by means of a metal strip or collar 29 on their projecting ends. The collar 29 holds the wires in place, is part of the lead for applying voltage to the electrode 28, and also acts as a heat'radiator. The wires of the grid-like electrode 28 may fit snugly into one of the plugs 27, but are preferably loose enough in the other plug to permit the wires to slide in the plug so that they can expand without distortion. The cathode 36 is a straight tungsten wire, like the cathode 15 in the fiat type tube shown in Figure 1, extending through the center holes of the plugs 21 and having its ends secured to supporting leads 3| which supply current to the cathode. These leads 3|, as well as the tubular electrode 26, may be supported from a stem of the usual type in any convenient way.

In accordance with my invention short wave tubes with a grid-like or perforated positive electrode may be made to generate oscillations of only one fundamental frequency by so constructing a tube that the times .of transit of the electrons on both sides of the positive .electrode are practically the same. This may be done by spacing the electrodes so that the ratio of the distance between the cathode axis and the positive electrode to the distance between the cathode axis and the limiting electrode is between approximately 1 to 1.5 and l to 1.75, de-

pending on the radius of the cathode. If in effect the cathode is flat, as in the flat type tube, the cathode axis is at infinity, and in this case the positive electrode should be approximately midway between the effective plane of the cathode and the limiting electrode; if the tube is cylindrical, .and the cathode is a very fine wire, practically a line, the ratio should be about, 1 to 1.75. As the size of the cylindrical cathode increases, the ratio becomes somewhat less, and'with filamentary cathodes of the usual sizes may be between 1 to 1.5 and 1 to 1.7, depending on the space charge conditions at the cathode. For example, in my cylindrical type short wave tube, above described, I have obtained good results with a tube having a tubular electrode 26 about 500 mils long and about 75 mils inner diameter; a grid-like electrode 28 about 45 mils in diameter and made of straight tungsten wires, each about 4 mils in diameter, and a cathode 30 made of a straight tungsten filament about 4 mils in diameter.

The circuit diagrammatically shown in Figure 6 indicates how a flat type of tube made up with two electrode assemblies, such as shown in Figure 3, mounted side by side, may be used to generate ultra high frequency oscillations. In this diagram the tuned output circuit of the generator is shown as a Lecher wire tuning circuit having two parallel Wires 32 and 33, which may be extended, if desired, to form a radiating antenna 35 and which are connected through the usual slidable bridge, which may contain a condenser 35. The two parallel wires of the Lecher system are connected to the fiat electrodes 22 and 23, and are supplied with the desired voltages through inductances 36 from voltage dividers 31. The cathodes I5, which may be connected in series or in parallel, are heated by a battery 38. The positive grid-like electrode l8 of the tube is biased through a voltage divider 39 to the desired positive potential with reference to the cathodes. The electrode l8 and the oathodes are preferably grounded through blocking condensers 43.

Measurements made on tubes constructed in accordance with my invention and connected to a Lecher wire system in the usual way have shown that when, for example, the tube is generating waves cm. long, and the line length of the system is varied by moving the bridge along the wires, well defined maxima of output occur at intervals of about 25 cm. in the travel of the bridge along the wires, with very little change in output between these maxima. Under similar conditions the tubes heretofore used produce several marked increases in output during the movement of the bridge over a distance of 25 cm. These results indicate that a tube made in accordance with my invention will oscillate strongly at a fundamental frequency only, and that any overtones and harmonics which may be present are too faint to be appreciable.

What I claim as new is:

1. An electron discharge tube capable of producing ultra high frequency oscillations and comprising a highly evacuated bulb enclosing a pair of fiat sheet electrodes mounted parallel to each other, a single straight electron-emitting filament approximately as long as said electrodes mounted between and parallel to them and nearer one electrode than the other, and a grid-like anode interposed between said filament and the more distant flat electrode with a portion in the space discharge path to said fiat electrode consisting solely of a pair of straight wires substantially as long as said filament and parallel to each other and to said filament and spaced from each other to leave between them an unobstructed gap in alignment with and at least as long as said filament and of a width substantially the same as the space between said filament and each of said wires.

2. An electron discharge tube capable of producing ultra high frequency oscillations and comprising a highly evacuated bulb enclosing a linear cathode, a fiat limiting electrode parallel to said cathode, an equipotential grid electrode having a lead extending outside said bulb and positioned midway between said fiat limiting electrode and said cathode with the portion in the space discharge path between said cathode and said limiting electrode consisting solely of two straight conductors parallel to said cathode and 'to each other and spaced apart a distance approximately equal to the spacing of each of safd conductors from said cathode, and a fiat backing electrode mounted on the opposite side of said cathode and parallel to said flat limiting electrode and spaced from it a distance of about 30 mils.

3. An electron discharge tube capable of producing ultra high frequency oscillations and comprising .a highly evacuated bulb enclosing a straight filamentary cathode, a fiat backing electrode mounted behind and parallel to said cathode, a fiat limiting electrode mounted in front of and parallel to said cathode and to said backing electrode at a distance of about 30 mils from said backing electrode, and an equipotential grid electrode having a lead extending outside said bulb and positioned midway between said limiting electrode and said cathode with an inter-.

mediate portion between said cathode and said limiting electrode consisting solely of two straight wires parallel to said cathode and to each other and spaced to leave between them a gap in registry with said cathode and of a width substantially equal to the distance between each of said wires and said cathodes.

PAUL D. ZOTI'U. 

